White is a symbol for innocence, purity, and virginity. It is a left over tradition from when daughters were the property of their father, to be given away, often with the bribe of a dowry. A daughter was no good if she was tainted or "ruined." A white dress showed she was pure enough to be married.
Many women still choose to go with the tradition but now that our society is changing it is becoming more socially acceptable to wear wedding dresses that are different colors.
I used to watch "Say Yes to the Dress" a lot and learned that in one culture it is traditional to wear a red wedding dress for excitement and passion. White was worn for funerals so the woman was really upset that she had a hard time finding the dress style she wanted in red fabric.